INDUSTRIAL AND ENGINEERING CHEMISTRY
902
mercury. This curve is of particular interest in that it shows how slight variations in moisture content may cause large variations in power factor of the resulting finished product. It also e3plains why preheating the molding material for a short time before molding will have such a large effect on the electrical properties of the finished product. Afterbaking the molded product improves these characteristics largely for the same reason. Any inherently good molding material can be ruined, as far as electrical qualities of the finished product are concerned, by exposing it for a relatively short time to atmospheric conditions involving high humidities such as frequently exist in molding shops, particularly if there are a few steam leaks in the lines or presses. On the other hand, many apparently poor electrical materials can be greatly improved by some treatment of the molding material, such as preheating, under conditions which tend to dry out any excess moisture. Generally, anything that affects one electrical characteristic of Bakelite materials will affect all of the electrical properties in the same way. This does not mean, however, that determining one of various electrical qualities will tell the whole story as to the suitability of the material for electrical uses. Apparently if only one property is to be measured, the power factor a t low frequencies will serve best as an indication of the insulation qualities of the material. Probably the dielectric strength tests could be considered the least satisfactory as a single test for determining electrical qualities. Even so, i t is necessary that all test conditions be carefully
***
Selenium in Soils
VOL. 27, NO. 8
controlled in order to secure satisfactory results from any testing procedure. Other types of tests may be applied to determine the suitability of an insulator for some particular use, but most of them are special tests designed to simulate working conditions which frequently cause failure of some kind in the field. Such tests are sometimes applied for determining acceptance or rejection of materials for that particular use and do not always indicate the general insulating qualities of the material.
Samples and Tests Curves shown are based on data obtained on various types of Bakelite materials molded under correct conditions. The laminated materials were standard paper-base laminated products. Samples BM-021 and BM-120 are standard, general-purpose, wood-flour filled, molding compounds. Sample BM-250 is a general-purpose, mineral-filled, molding material with higher heat resistance and lower moisture absorption than the wood-flour filled materials. Sample XM262 is a low-loss, mineral-filled compound used for special electrical purposes. Sample XM-1000 is another specialpurpose, cellulose-filled material with improved characteristics such as arc resistance and higher direct-current resistance a t elevated temperatures. All of the results of tests given were obtained by the standard methods of measurement of the American Society for Testing Materials unless otherwise specified. R E C E I Y EMarch D 8, 1935.
#***
In Relation to Its Presence
in Vegetation HORACE G. BYERS
AND
HENRY G. KNIGHT
Bureau of Chemistry and Soils, Department of Agriculture Washington, D. C.
A
T A MEETING of the Associationof Offi-
cial Agricultural Chemists in Washington on October 30,1934, one of the authors presented an address on “The Selenium Problem.” Attention was called t o various phases of this new agricultural question and in particular to the following facts which were developed as a result of cooperative effort by various bureaus of the Department of Agriculture and the experiment stations of South Dakota and Wyoming. Selenium has been shown to be present in a wide variety of plants growing upon certain soil areas. It is present in the plants in concentrations which range from traces up to quantities which are lethal (to animals). I n many cases the selenium present produces chronic diseases which may ultimately cause death. Quickly lethal results and chronic disease have both been produced by addition of inorganic selenium compounds or of seleniferous vegetation to the normal diet of both large and small animals. It has been found possible to trace the source of the selenium in the plants to the soil upon which they are grown and to certain shales which are the parent materials of the seleniferous soils. From the available data it seems probable that the primary source of the selenium is iron pyrites or other sulfide ores containing selenium. I t should therefore be present to
Selenium occurs in soils to a varying extent over extremely wide areas. In certain limited areas it is present in quantities sufficient to produce toxic vegetation. The quantity of selenium in vegetation grown upon a soil depends not alone upon the concentration but also upon a variety of other factors which include the plant species, the composition of the soil, the moisture relations in the soil, the stage of development of the plant, and the portion of the plant examined. The nature of the general selenium problem is briefly discussed. some degree in all soils derived from pyritiferous materials in areas where rainfall is not sufficient to leach it from soils. The serious character of the situation as well as the novelty of the problem has attracted widespread interest. It was also pointed out in the address mentioned that, while there exists a wide variation in the quantities of selenium in vegetation grown upon seleniferous soils, the variation is not due alone to differences in the soil content. It is influenced by a number of factors, among which are the sulfur content of
AUGUST, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
soil and the species of plant grown. It is to certain developments in this connection that attention is directedin this paper.
Effect of Plant Species and Soil Composition An interesting illustration of the influence of the kind of plant upon selenium absorption is the following: In a series of 122 surface soil samples, which represent a coniposite picture of the soil of a single square mile in western South Dakota, the selenium content varied between 0.7 arid 13 parts per million and averaged 4.8 p. p. m. From this area 50 samples of little bluestem had a mean selenium content of 0.8 p. p. m., and 12 samples of western wheat grass had a mean content of 20 p. p. m. The mean content of all the vegetation (137 samples) mas 36 p. p. m.; yet a single saniple of wild aster contained the surprisingly large quantity of 2670 parts. I n another section 15 samples of growing wheat had a mean selenium content of 21 p. p. m., while 19 samples of needle grass had a mean content of but 6 p. p. m. Perhaps the kind of result brought about through selective absorption by plants is best shown by the plots a t the Duel1 Ranch near Laraniie, Wyo. These plots were sampled by T. D. Rice, of the Soil Survey. Soil samples were taken a t the points indicated in the figure, and each plot mas sampled to secure a composite of three kinds of vegetation growing on each plot. The kinds of vegetation were Astragulus bisulcatus, Astragulus Missouriensis, and native wild grasses.
SELENIUMCONTENTOF SOILSAND VEQETATION MENTAL PLOTS ON THE DCELL RASCH,ALBANY
FROM
Soil, S 6 in. Soil, 0-6 In. Mixed grasses 4 . Massozrriensis 1.bzsulcatue
3.0 2.0
4856 4861 4843 4844 4845
B2N B2S B2a B2b B2c
Soil, 0-6 in. Soil, 0-6 in. Mixed grimes A. Missouriensis A . bisculcatus
1.5 2.0
4857 4862 4846 4847 4848
B3N B3S B3a B3b B3c
Soil, 0-6 in. Soil, 0-6 in. Mixed grasses A . .Missouriensis A . bisulcatus
2.5 2.0
4858 4865 4849 4850 485 1
B4N B4S B4a B4b B4c
Soil, 0-6 in Soil, 0-6 in. Mixed grasses A. Missouriensis -4. bisulcatus
2.0 2.0
4859 4864 4862 4853 4854
B5N B4S B5a B5b B5a
Soil, 0-6 in. Soil, 0-6 in. Mixed grasses A, Mmouriensis A . bisulcatus
2.5 1.5
1E
Young wheat at Beath's plot A . bisulcatus among wheat Grain from 1E when grown
2E 3E
EXPERIMENTAL I W OF WHEAT
EXPERI-
Selenium, P. p. m. Soils Vegetation
Field NO. BIN B1S Bla Blb Blo
5652 5908 12551
which, like A . Missourensis, have but little tendency in this direction. There also appear to be several species of aster which show marked absorptive differences. That these differences may be due to different root distribution, and that high absorption may be occasioned by the fact that d e e p feeding plants reach points of higher selenium concentration have been considered. A study has been made of the general differences in root distribution between a group of highly absorptive plants and a second group of low absorbers. Sod-forming grasses have a predominant surface root system and many of the absorptive group have deeply penetrating roots, but particularly in the season just passed, the drought compelled all the plants to go deep for water and it is not deemed probable that root distribution accounts for the observed differences. It appears to be really selective absorption.
C07JNTYJ W Y O .
Lab. NO. B4855 4860 4840 4841 4842
Material
903
...
, . .
...
... ... ... ... ... ...
... ,..
...
... ...
... ... .... ..
.. 1o:o
2.5 1000
.. 1o:o 4.0 800 ,.
1o:o
5.0 2050
4:0 2.0 1460
ii:o 2.0 940
45.0 1110 2.0
I n the table it is to be specially noted that the selenium content of the soil varies within narrow limits and averages 2.1 p. p. m. The carefully sampled grasses have a mean content of 10 p. p. m., the Astragulus iVissouriensis, 3.1 p. p. m., and the Bstragubus bisulcatus, 1250 p. p. m. A sample of young wheat growing in the Beath plot adjoining the others has 45 p. p. In. and A . bisulcatus growing among the wheat has 1110 p. p. m. A sample of grain from the matured wheat upon this plot has a selenium content of but 2 p. p. m. Relations of the type shown by these data are abundantly shown by other samples from other sources. Not only are several species of Astragulus known which, like A . bisulcatus, can absorb large quantities of selenium, but there are other species
The selenium content of the soil in any given profile varies with the depth, but by no means is the distribution uniform in character. Many profiles have been examined. In some instances the concentration indicates a zone of selenium accumulation in or near the zone of sulfate accumulation. The exceptions to this rule are numerous. In some profiles the selenium concentration appears to diminish with depth. It would seem that, when plant roots reach zonei of high selenium content, the content of the plants should ala0 increase. This appears in general t o be the case, but other influences, some of which are mentioned below, mask the relation,
Effect of rMoisturein Soil Karrer (1) of the Bureau of Plant Industry has discovered that the presence of available sulfur comp3unds in water cultures and of sulfur or sulfates in pot tests markedly affect the quantity of selenium absorbed by plants from a given selenium concentration. The areas of soils studied so far are areas in which sulfates, chiefly calcium sulfates, are p e s ent in some portion of the soil profile. In one area, an irrigation tract, the soil contains quantities of selenium which in other areas are quite sufficient to produce toxic vegetation; yet in this area no toxic vegetation is found except in a few samples of the plant species of the highly concentrating groups of plants. Examination of the soil solution and of the irrigation water from this area shows the presence of from fifty to several hundred fold greater concentration of sulfur, as sulfates, than of the total selenium in the soil. In another area high sulfate in the soil extract corresponds with relatively low selenium content in the vegetation. I n still another area low sulfate in the surface soil is accompanied by high selenium content in the vegetation. Incident to the study of irrigation areas (a study far from adequate as yet) subsurface drainage (tile drains) appears to lower the selenium content of the soil. In one case, water
904
ISDUSTRI.4L AXD EKGINEERING CHEMISTRY
from a drain in operation 16 years in a gumbo soil has a m a l l selenium content, while water from one operating for a year only has fifteen times as great a selenium content (0.08 and 1.2 p. p. in., respectively). I n another area of less dense foil, a drain operating for 2 11-eeks only has uater with 1.98 p. p. in. of selenium, while a near-by drain operating for an unknown period contain- 0.07 p. p. m. It appears possible, therefore, that a remedy for the injury due to selenium in foilq may be found through irrigation where irrigation is practicahle. It is hoped that the coming season may give opportunity for gaining definite information on this point. Since in many soils affected by selenium the zone of sulfate accumulation is reasonably near the surface, there i4 considerable interest attached to the question of what occurs when soil moisture moves freely from one portion of the soil profile to another. The a n w e r from the field has not been possible since there has been no opportunity to study the area except under drought conditions. It is hoped that opportunity may also be found in the coming season for a study of this question.
Distribution of Selenium in the Plant T h a t plants absorb selenium from seleniferous soili it clearly demonstrated, but the fate of the absorbed selenium is largely unknown. It has been already mentioned that young wheat plants on the Beath plot contained 45 p. p. m. of selenium 17-hile the matured grain contains but 2 p. p. m. This relation appears to be qualitatively general. Young plants contain more selenium than matured seedf. Fragmentary information indicates that green leaves contain more selenium than do portions of the plant higher in cellulose. Robinson (a)found that wheat leaves in a given sample contained 40 p. p. m. while the corresponding wheat stalks contained 12 p. p. m. The wheat grain from this sample contained 8 p. p. m. Definite selenium compounds have not as yet been isolated from either soil or vegetation, but certain definite facts are available. Very early in the course of the selenium investigations E. bI. ?;elson was able to show that the toxicity of seleniferous wheat is largely concentrated in the wheat gluten, and \Ir. 0. Robinson showed that in thiq wheat the concentration of selenium was 10 p. p. m. while in the gluten it was 80 p. p. In. In another wheat sample Daniel Ready found 25 p. p. in. in the whole wheat, while the gluten contained 180 p. p. in. and the bran 25 p. p. m. Therefore, the seleniferous compound in wheat Seems to be a portion of the protein fraction and as such is insoluble in water and ether and nonvolatile with steam. J. F. Couch, of the Bureau of Animal Industry, waq able to show that green vegetation (dstrug7tlus bisulcatus) may be freed almost wholly from selenium by digestion with hot water and that a part of the leached selenium is volatile with fteam and soluble in ether. Therefore, plants appear to metabolize a t least two distinct types of organic selenium compounds, and the presence of inorganic selenium compoundq in plants is also possible. Although no specific effort has been made to identify the selenium compounda present in soils, two types of compounds seem t o be present. A seleniferous soil may be extracted with water and selenium demonstrated in the extract. If, then, the organic matter of the soil is extracted with ammonium hydroxide, selenium is found in the extract. It is extremely probable also that a part of the soil selenium may be present as the free element Although it has been demonstrated that seleniferous soils are of rather wide distribution and in quantities sufficient to produce toxic vegetation, it has also been demonstrated that, while all kinds of vegetation grown on these soils contain some selenium, by no means is all the vegetation toxic. It has also been s1ion.n that in humid areas certain soils contain
VOL. 27, h-0.8
selenium, but to the present time no area has been found containing quantities which promise toxic vegetation. So far, the soils found to contain serious quantitiej of selenium are heavy clays which have been developed from shale depo-itof the Cretaceous period. S o sandy soils have been found to contain more than traces of this element. For the most part, seleniferous soils have been identified only in marginal or submarginal areas as respects normal agricultural operations.
The Selenium Problem The whole selenium question bristles with unsolved problems, both of theoretical and practical importance. Among these are the identification of the compounds and the tracing of their development in plants, and the determinat’ion of the metamorphoses which occur in the animal body and in their return to dust. It is true that quantities of toxic foodstuffs have been produced and marketed and, unless preventive measures are taken, will continue to be produced and marketed. It seein$ however, that no serious concern need be felt except in the areas concerned. In the general market it is improbable that any serious concentration of toxic food is likely to reach any individual. Within the toxic areas serious injury to stock raising has been encountered and human injury, especially to the young, may have occurred. However, this bureau has deemed it to be in the interest of the general welfare as well as in the interest of the local public to move slowly on sure ground rather than to take hasty and unwarranted steps which may be retraced with difficulty. The conditions in question have obtained for many years. All that is new is the discovery of the cause of certain known results. Knowing the cause, remedial measures are more readily established.
Literature Cited (1) Karrer, h. M . , Science, 78, 560 (1933); 343 ( 1 9 3 4 ) . ( 2 ) R o b i n s o n , IT.
J.
A g r . Research, 49,
O., unpublished data.
RECEIVED April 27. 19335. Presented before the Division of Biological Chemistry a t the 89th Meeting of the American Chemical Society. S e a Tork, N. P., hpril 22 t o 26, 1935.
Correction Arithmetical errors have been discovered in one of the illustrative examples included in our paper on “Distillation and Absorption in Packed Columns” [IND.EXG.CHEM.,27, 255-60 (1935) 1. The following corrected figures should be substituted for the table appearing at the bottom of page 257 as part of the caption of Figure 4: X
P
P*
P
- P*
l / ( ~ P * ) PO 14.1 0.85 17.4 0.878 0.903 22.3 29.3 0.926 4 0 . 0 0.949 5 7 . 5 0.969 8 0 . 8 0.985
pa/
Pd P ~ ( P- P”
0.886 0 0i89 0 0711 0 . 3 0 0,1500 0.906 0 0574 0 25 0 , 1 2 2 2 0 0658 0 .925 0 0526 0 0448 0 . 0 9 7 4 0.20 0.944 0 0341 0 0394 0 . 1 5 0.0735 0.961 0,0250 0 0263 0.10 0.0513 0,978 0 0132 0 0174 0 05 0,0306 0.991 0 . 0 1 0.01500 0 00263 0.01237 Area under curve = 4.40 transfer units: by the approximate (Equation 18) I I = 4.12 transfer unite.
14 18 22 29 40 58 81
7 0 9 9 5
0 3
method
T. H. CHILTON AND A. P. COLBURX E.I .
D U P O X T D E ~ E Y O U R J&
COMPANY, INC., WILMISQTON, DEI, J u n e 5, 1935
